1. Field of the Invention
The invention relates to a formed article for a vehicle body structural member that is a component of a vehicle body, and in particular, to a formed article for a vehicle body structural member that is formed of a steel plate with tensile strength higher than that of a normal steel plate (high tensile steel plate) or a steel plate with a tensile strength further higher than that of such a high tensile steel plate (super high tensile steel plate).
2. Description of the Related Art
Japanese Patent Application Publication No. 2006-281312 (JP-A-2006-281312), for example, discloses a vehicle body structural member, such as a roof side rail outer reinforcement that is disposed along a roof side of a vehicle body, and a center pillar upper or lower reinforcement that is disposed along a center pillar, the vehicle body structural member being formed to have a hat-shaped cross section that is open toward one side in the lateral direction of the vehicle. A vehicle body structural member with such a configuration is formed by drawing (pressing) a flat steel plate.
In recent years, a so-called “high tensile steel plate” that has a tensile strength higher than that of a normal steel plate is used for such a vehicle body structural member. Moreover, consideration is given to the use of a so-called “super high tensile steel plate” that has a tensile strength further higher than that of the high tensile steel plate.
Meanwhile, with regard to the roof side rail outer reinforcement, the center pillar upper reinforcement, the center pillar lower reinforcement, etc. the height of the cross section, that is, the height from the flange portion to the top wall portion of a hat shape, may vary at certain positions along the longitudinal direction (the vehicle height direction when the vehicle body is assembled) according to the design, the mechanical strength, the rigidity, etc. of a vehicle.
If the flat steel plate is formed into such a shape by drawing (pressing), in the top wall portion, compressive residual stress occurs in the width direction in the portion in which the height of the cross section is high, and tensile residual stress occurs in the width direction in the portion in which the height of the cross section is low. When such residual stress occurs, the dimensional accuracy is deteriorated, that is, for example, a “twist” occurs around an axis extending in the longitudinal direction of the top wall portion, and/or “wrinkles” occur in some portions, after forming the member.
The problem, such as the deterioration of the dimensional accuracy, that occurs after forming becomes severe when a “high tensile steel plate” is used, which has a tensile strength higher than a normal steel plate, and becomes more severe when a so-called “super high tensile steel plate” is used, which has a tensile strength higher than a high tensile steel plate, and it becomes difficult to obtain a desired shape.